疲劳载荷作用下铝镁合金中的粗滑移区的形成

过去关于铝和铝铜合金的研究工作指出,在疲劳载荷过程中的能量消耗(△E)所发生的变化可以分成两个不同的阶段。第一阶段相当于位错的被钉札,在第二阶段里△E的再上升则表示已有粗滑移区出现。为了进一步验证这种看法,在本文中用含镁量为0.52,0.91,3.46和5.15％的铝合金进行了扭转疲劳试验,测定了经过各种应力循环数N以后的滞后迴线的面积,从而算出在每次循环中的能量消耗△E。在疲劳载荷经过不同循环数后,试样表面进行金相观测的结果指出,对于所用的合金而言,滑移痕迹的变化都表现出两个明显不同的阶段。在第一阶段里,常常观察到几组细而直的滑移线均匀分布在一个晶粒内。在第二阶段里,某一组滑移线变得集中而粗化成簇。一般而言,在镁含量较低(0.52,0.91％)的合金里,以及当扭应变较大时,粗滑移区出现得较早。将所观察到的△E-N曲线的变化与粗滑移区的出现做了比较,并且考虑到在疲劳载荷过程中第一阶段和第二阶段的△E可能发生重迭的情况,指出了试样里出现粗滑移区可以引起△E在疲劳后期的再上升。这与过去关于铝和铝铜合金所得结果相合。本文还讨论了位错在疲劳载荷第一阶段里被溶质原子气团所钉札的状态与粗滑移区的随后形成的联系。

OCCURRENCE OF LOCALIZED SLIP REGIONS IN ALUMINUM-MAGNESIUM ALLOYS UNDER FATIGUE LOADING

Previous studies on aluminum and aluminum-copper alloys have shown that the change of energy loss (△E) under fatigue loading can be divided into two stages. The first stage corresponds to the pinning of dislocations and the rise of △E in the second stage is associated with the occurrence of localized slip regions in the specimen. In order to confirm this viewpoint, torsional fatigue experiments were carried out on aluminum-magnesium alloys containing 0.52, 0.91, 3.46 and 5.15% of magnesium, and the area of the hysteresis loop (stress versus strain) after various stress cycles N was determined, from which the energy loss △E in each cycle was calculated. Results of metallographic observations on the specimen surface after various stress cycles show that for all the alloys used, the change of slip marks exhibits two distinctly different stages. In the first stage, several systems of fine and straight slip lines are often observed, and the slip lines are found to be distributed uniformly over a grain. In the second stage, some of these slip lines are thickened up to form localized slip regions. In general, such localized slip regions occur earlier in alloys with lower magnesium contents (0.52, 0.91%) and under higher applied maximum torsion strains.A comparison was made between the observed changes of the △E-N curves and the occurrence of the localized slip regions. After proper account is taken of the possible superposition of the△E's occurring in the first and second stages, it is shown that the occurrence of localized slip regions in the specimen can give rise to an increase of △E in the later stage of fatigue loading. This agrees with the results previously obtained in the case of aluminum and aluminum-copper alloys.The correlation between the state of the pinning of dislocations in the first stage of fatigue loading and the subsequent occurrence of localized slip regions is discussed.